Almost every fornight for more than a year a group of highly organised and meticulous career criminals put on hooded white overalls and picked up sledgehammers and CS gas – the tools of their trade

Their targets were high-end jewellery shops across England. In 16 months they plundered more than £3.5m in gems and watches after smashing their way in and gassing staff.

The raids would usually last 60 seconds or less, with the gang using guns and the CS spray to overpower any resistance.

Several of the daylight attacks saw shop workers or passers-by attacked with "gratuitous" violence.

Yesterday, 17 of the gang's 30 members were jailed for a total of 141 years after a court heard how Scotland Yard's flying squad caught them in an elaborate trap. Tomorrow the remaining 13 will be sentenced. They include the gang's only woman.

The police operation outside the Mappin and Web shop in Guildford, Surrey, is the biggest in the flying squad's history. A police officer videoed two men wearing white overalls smashing the windows of the store with sledgehammers as detectives pulled up to arrest them.

A second undercover operation led to the arrest of four more men as they prepared to raid David Dudley jewellers in Marlborough, Wiltshire.

Detective Inspector Jason Prins, of the Met's flying squad, said: "We targeted these violent individuals who have shown they were prepared to use significant force to steal large quantities of jewellery.

"The incidents were terrifying for the staff members and their customers who were caught up in them. It is our duty to protect people from this level of violence.

"These defendants operated as an organised criminal network, meticulously planning and carrying out the most brazen daylight raids."

Most of the gang lived in the home counties. They researched their targets on the internet before staking them out.

Gang member Lee Fairbank, responsible for at least 31 previous offences, was sentenced to 15 years. Kingston crown court heard that two other leading members of the gang, Anthony Bado and James Stewart, had taken part in one heist on a diamond shop in Antwerp where a member of the staff was kicked to the ground, given an electric shock with a Taser gun and smashed over the head. As they fled the robbers fired CS spray at staff and passers-by.

Judge Richard Southwell said: "The raid was characterised by a serious degree of violence which was vicious and largely gratuitous.

"Indeed the safe was locked and you couldn't get into it and there was no need for any violence. All you had to do was depart."

Stewart was jailed for 14 years and Bado for 12 years.

Southwell said: "The total number of attacks on shops occurred was no less than 34 and the offending stretched between January 2008 and May 2009.

"There was an average of attacks on shops twice a month.

"A gang of five or six men in the main would target a shop, usually in a provincial town but sometimes in London or on the outskirts, and very often a reconnaissance trip would be conducted.

"The gang always travelled to the premises in two or more cars, sometimes arriving the night before, sometimes laying down the getaway car the night before."

In at least two of the attacks shop staff were threatened with guns. "Many people, the staff, members of the public who witnessed these events, spoke of their fear, their stress, and sometimes their anger," the judge said.

"Others talked of their anxiety and sleeplessness. Many have sought counselling to come to terms with what has happened."

Usually in cases involving multi-million pound thefts prosecutors would seek a confiscation order to recover the proceeds of crime. But prosecutor Brian O'Neill said no such order would be made because the criminals had "lived their lives in the fast lane", blowing the money they made on drink and drugs.